I was told by a couple of people (one being my Japanese teacher in these parts) that you shouldn't use "sensei" when referring to yourself. Kyoushi is supposed to be more appropriate. But anyway, I was actually reading this during a free period today.

Maybe someone walks into a class and asks if you're the foreign exchange student.
私が先生です - "I am a teacher", with the emphasis on "I", so maybe becoming "I'm the teacher".

Does that work, or am I screwing up understanding even more?

EDIT: edited for being so confused I got it backwards

Well sort of. You've got the basic idea that the best way to explain it is using emphasis. Think of it this way: は emphasizes the action of a sentence, whereas が emphasizes the subject itself. In your previous example, the particle は would be most appropriate in both cases, since the sentence's emphasis is on the verb, in this case the act of being a teacher. However, I love your translation of 私が教師です (watashi, kyoushi) - "I'm the teacher!" Imagine some little sannen smartass is acting up in your class and you want him to quiet down. "おい！静かにしろ！俺が教師だ！" "Hey, shut up! I'm the teacher!" or something similar. The sentence's emphasis has moved to the subject - you're not stressing the action that's being done, but rather who is doing it. Another good example would be ”あの人は野球選手です。” ”へぇ？誰が？” ("That person is a baseball player." "Wha? Who is?") In the first sentence the emphasis is on the act of being a baseball player, whereas in the second the emphasis moves to the person who's performing that act. Capisci?

Of course, there's also the old topic/subject distinction. Look at "私は車があります。", "I have a car." In this case, the subject's topic, 私, is different than its grammatical subject, 車. This sentence translates most literally as "Talking about me, a car exists." See the difference between topic and subject? This only really applies to sentences where both particles are present. In a sentece with either one or the other, you're more likely dealing with the case above.

thanks for the help. I've focused almost entirely on coversational Japanese, and completely ignored studying grammar. So I can have 20min conversations, but I have to stick mostly to using simple sentences, lots of verbs and adjectives (and slang of course, あり得ないよ), and using English prepositions. Makes for interesting and confusing conversations.

* And the Lord said unto John "Come forth and receive eternal life." But John came fifth and won a toaster. *

One thing that helps is to remember that in its simplest sense, GA hangs out in the same realm as WO. GA tells you what's doin/feelin the verb and WO tells you what cops the brunt of the verb. There are other particles in this dimension but they won't be mentioned here :x

WA lives in another dimension. It can highlight any object, subject, place, time or anything you wanna focus on. I'm sure you've seen NIWA, TOWA and DEWA particles before. That's WA flying in from its magical dimension to do its job and remind the speaker of what you're talking about. The thing is, when it steps up to remind you of who's doing/feelin the verb or whats copping the brunt of the verb, you don't say GAWA or WOWA. You just say WA.

It's useful for students to remember that GA and WO both get replaced by WA quite regularly. Not just GA. If you can understand why, then you'll be better at seeing just how unique WA is and when u use it.

Aside: My friend tells me this helped him.

neko HA hashirimasu
can be thought of as meaning the same as
neko HA neko GA hashirimasu.
(As for that cat I told you about before, yeah, that cat's running)

except, you would never say the 'neko GA' part coz you can already assume that its the thing doing the verb. The japanese are masters at omitting anything that is readily understood and you would never say something ludicrous like 'GAWA'.

There. Now you're even more confused than before, which is good because at least now you're probably thinking about just how different WA, GA (and WO) were from what you might have been guessing at...
:smt009

I think this is a good example to use. Above I just said "you look pretty today". But when I used は I am actually implying "as for today, you look pretty" meaning that today you look pretty but on other days you may not be pretty. Don't say this to the ladies.

今日がきれいです。

Now I am also saying you look pretty today. But I do not have the added implications for other days. I'm just talking about today and only today.

Am I making any sense?

You see, gentlemen, a pimp's love is very different from a square's...
(郷に入っては郷に従え.)